1. Field Of The Invention
This invention relates generally to electrostatic precipitators (ESPs) used to precipitate particulate matter from exhaust gases onto collection substrates by electrostatic charge, and more specifically relates to the collection substrates (collecting electrodes).
2. Description Of The Related Art
Industrial electrostatic precipitators (ESPs) are used in coal-fired power plants, the cement industry, mineral ore processing and many other industries to remove particulate matter from a gas stream. ESPs are particularly well suited for high efficiency removal of very fine particles from a gas stream. Specially designed ESPs have attained particle collection efficiencies as high as 99.9%. However, conventional ESP collection efficiencies are at their lowest values for fine particle sizes between 0.1-1.0 .mu.m. Additionally, conventional ESPs cannot address the problem of gaseous emissions or gas-to-particle conversion.
In 1997 the Environmental Protection Agency (EPA) proposed new air quality standards for fine particulate matter. The focus of the regulations is the emissions of fine particulate, i.e., particles below 2.5 .mu.m in diameter (PM2.5). These particles more readily enter the human respiratory system.
In a typical conventional ESP, vertical wire electrodes are placed in the midsection of a channel formed between vertical parallel collector substrates. The heavy, typically steel, plates are suspended from a support structure that is anchored to an external framework. Commonly, ten of the single precipitation channels constitute a single field. Industrial precipitators have three or more fields in series. An example of such a structure is shown and described in U.S. Pat. No. 4,276,056, 4,321,067, 4,239,514, 4,058,377, and 4,035,886, which are incorporated by reference.
A DC voltage of about 50 kV is applied between the wire electrodes (discharging electrodes) and the grounded collector plates (collecting electrodes), inducing a corona discharge between them. A small fraction of ions, which migrate from the wires towards the plates, attach to the dust particles in the exhaust gas flowing between the plates. These particles are then forced by the electric field to migrate toward, and collect on, the plates where a dust layer is formed.
In dry ESPs, the dust layer is periodically removed from dry ESPs by hammers imparting sharp blows to the edges of the plates, typically referred to as "rapping" the plates. When ESPs are rapped, the dust layer is supposed to drop vertically downward from the plates due to a shear force between the plate and the parallel dust layer. However, due to initial imperfections and in-plane compressive forces, plates tend to buckle when rapped as shown in FIG. 5. The compressive loading in this so-called normal-rapping mode generates fast propagating stress waves, along and across the plate, that are manifested in large lateral amplitudes (displacements) of the plates in the direction normal to the plate surface.
Several complications result from the rapping process. Due to buckling of the plates, imparting a force on the plate will cause some of the dust to be expelled away from the plate. This dust may then be re-entrained into the gas flow, where it may or may not be removed by downstream collection plates. The disruption of the ash layer caused by the rapping force, combined with buckling of the plate, tends to break the ash layer into small pieces. Smaller ash pieces are more susceptible to re-entrainment than large pieces, which tend to stay in the laminar boundary layer of gas flow that exists next to the collection plate and then slide down into the collection hopper.
Conventional collector plates are stiffened with ribs aligned along the direction of hammer impact force to reduce buckling and stresses and fatigue of the plates. These ribs support the plates during rapping to reduce the amplitude of plate vibrations that cause dust to be broken into clouds. However, such ribs greatly decrease the smoothness of the gas flow through the channels. It is highly desirable that the gas flow between collector plates be uniform. Turbulence can decrease collecting efficiency several times and will result in a less uniform layer thickness. Turbulence causes some of the dust that is broken into a cloud to continue along in the gas stream, and this dust is re-entrained into the gas stream.
Dust that re-entrains into the gas flow stream as a result of rapping in the upstream fields may be re-precipitated in the downstream fields. However, dust precipitated on the most downstream field in dry ESPs does not enjoy this privilege, and therefore re-entrainment occurring at this field becomes a critical factor in the overall collection efficiency of the dry ESP.
Studies on full-scale dry precipitators suggest that re-entrainment of fly ash due to rapping accounts for 30% of the time averaged penetration for cold-side units and as much as 60% for hot-side ones. In the last few decades, driven by regulations requiring mass collection efficiencies on the order of 99.8% and higher, the design of precipitators has evolved towards units of much larger specific collection areas and higher cost. For that reason the issue of controlling the rapping re-entrainment has become critical. The overall goal of dust rapping should be to efficiently remove the precipitated ash, with minimal re-entrainment.
The problem of rapping to remove the dust layer is formidable. The dust layer can be up to 1 cm thick, and it should detach from the typically 10 m long vertical plate bounding the turbulent gas flow and slide down into hoppers with a low re-entrainment. In order to successfully rap, the dust layer should fracture into pieces which are as large as possible. Furthermore, the pieces should, while falling, remain as close as possible to the plate where they are "hidden" in the gas-flow boundary layer, where the gas flow velocity is low. However, due to buckling and turbulence, rapping tends to result in re-entrainment.
In general, dry ESPs will also have difficulty in meeting the aspects of the PM2.5 standards that relate to gas-to-particle conversion. In gas-to-particle conversion, particles 0.1 .mu.m or smaller that form from SO.sub.2, NO.sub.x, and other gaseous materials, grow rapidly by coagulation or nucleation on smaller sites. Particles grow slowly beyond 2 .mu.m, since diffusional effects are greatly reduced.
There are two reasons dry ESPs are not effective at controlling gas-to-particle conversion. The primary reason is that ESPs using metal collecting plates do not effectively remove gaseous pollutants that coagulate to form the sulfate and nitrate particles. Second, ESPs are inherently less effective in removing particles in the 0.1 to 1.0 mm range, which is in the size range of potential nucleation sites for growth of particles from gaseous material. As a result, dry ESPs do not effectively reduce the source of much of the small particle emissions from power plants, and will have problems meeting the PM 2.5 requirements.
Current work in this field offers the likelihood of converting much SO.sub.2 to SO.sub.3 inside the ESP by electron attachment. In this process, free electrons are formed in a nanosecond-pulsed corona. A wire electrode is charged, usually via negative DC voltage, in a rapidly oscillating manner. The pulsing enhances the corona effect, ionizing more gas and producing more free electrons for beneficial interaction with NO.sub.2 or SO.sub.2 molecules. Two mechanisms have been proposed to explain how this process leads to the removal of SO.sub.2. One is via direct electron attachment forming a charged SO.sub.2 molecule for direct collection. The other is through the formation of SO.sub.3 via the formation of ozone, O.sub.3. SO.sub.3 rapidly forms H.sub.2 SO.sub.4 (sulfuric acid) via the reaction H.sub.2 O+SO.sub.3.fwdarw.H.sub.2 SO.sub.4. The acidic environment leads to increased corrosion of the steel plates and ductwork. Therefore, electron capture and pulsed-corona techniques will require that collectors be made from materials that resist chemical attack by sulfuric acid.
A different type of ESP, which uses water, is called wet ESP. In such a system, a vertical plate is covered by a film of flowing water passing from the top of the plate to the bottom. The flowing water acts as both the collecting electrode and the ash removal mechanism. Wet electrostatic precipitators offer the advantages of fewer re-entrainment losses, the ability to collect reactive gases and elimination of rapping. However, because of the oxidizing effect of water, the use of metal plates is prevented by the induced corrosion. Disposal of the ash-laden water is also a problem.
In addition to the problem of corrosion associated with wet ESP, the substrate material used to transport the water film has to be consistently and continuously wetted to prevent the formation of "dry spots", which are typical for steel plates in wet ESPs. Otherwise, ash can accumulate on the dry spots and prevent further capture of particulate matter and gases in those regions of the collecting surface.
Any ESP that is expected to be implemented in response to the new EPA requirements should be able to be retrofitted in many industrial applications in which conventional, inefficient ESPs are currently operating. Inexpensive retrofitting of existing dry ESPs to meet the new, increasingly stringent particulate emissions standards is of great interest for a number of industries. In response to those interests, Chang and Altman of EPRI have recently evaluated fine particle control technologies, for particles &lt;2 .mu.m, and have conducted detailed economic evaluations of retrofit methods for improving the particle control effectiveness of existing ESPs.
Three promising options, all of which are devices added downstream of the existing ESP, have been evaluated. All of them have the potential to reduce particle emissions to &lt;0.01 lb/Mbtu at the stack. A cost analysis of seven combinations indicates that a stand-alone wet ESP retrofit would cost the most (2.5 mills/kWh), while retrofit of the wet ESP into the last field of an existing dry ESP has the lowest cost (1.2 mills/kWh). The retrofitting option also gives a new chance for dry ESPs, which in combination with the wet section (hybrid ESPs) may be used to exploit the best properties of both. For example, a hybrid ESP can optimize particulate collection by using the dry section to remove 95% or more of the particulates, while the wet portion could be used to facilitate the pulse-corona technique and to eliminate the re-entrainment loses. It is clear that hybrid ESPs offer a possibility of reducing the water contamination from wet ESPs to a minimum.
Therefore, the need exists for a lightweight electrostatic precipitation collector substrate that is conductive, resists corrosion due to water and/or acid environments and can be wetted. The collector should also be easily retrofitted to existing ESP systems.